I am in the Pacific Northwest, and I can buy those at Wesco Auto body supply? Looks like there is one in Milwaukie. Maybe see if they have them? (503) 722-2827
Just Google the pt. number. Lots of places show available. That's one expensive backing pad!! You can get a 7" dia. pad with 5/8-11 ctr at Harbor freight for about $12 https://www.harborfreight.com/7-in-fiber-disc-backing-pad-58748.html